The present invention relates to helmets with a protective visor and especially those equipped with an optical image-projection system making it possible to display in front of the helmet wearer""s eyes, a synthetic image superimposed onto his view of the external world.
This sort or helmet must comply with a certain number of restrictions, especially size restrictions since it is intended to be used in limited spaces such as the cockpit of an aircraft or of a helicopter and weight and kinetic moment restrictions in order to limit fatigue in the neck of the wearer. These restrictions lead to the components of the image projection system being lightened as much as possible and to them being placed as close as possible to the center of gravity of the wearer""s head.
The components of an image projection system for a helmet are in general dispensed with, with the exception of the optical combiner making it possible to superimpose the projected synthetic image onto the scenery in front of the helmet wearer""s eyes, at the upper periphery of the helmet since they should not encroach upon the visual space of the helmet wearer, nor impinge on the lower periphery of the face of the helmet wearer, which is reserved for any respiration equipment (oxygen mask). The synthetic image generated by the image projection system is therefore applied to the combiner placed in front of the helmet wearer""s eyes from an optical system occupying the forehead region.
Moreover, the helmet is still equipped with one or more adjustable protective visors, for example, a visor for mechanical protection against wind, dust, etc. and a visor for optical protection against the sun or laser attack. This or these movable protective visors are completely detachable or they retract by pivoting above the helmet wearer""s head, passing above the components of the image projection system placed in the forehead region. In order to take up less space when they are moved into place and retracted, it is usual to give the visors a generally spherical shape and to articulate them on the helmet about an axis of rotation placed in the forehead region of the helmet and passing through the center of their sphere. In addition, for ease of manufacture, the visors, which are generally obtained by thermoforming a transparent plastic sheet of uniform thickness, often have a final concentric bispherical shape with a constant thickness and two faces: a spherical and concentric internal face and external face.
The positions of the helmet wearer""s eyes are offcentered laterally and downward with respect to the center of the spheres of the external and internal faces of the visors, by angles exceeding 20xc2x0. These offcenterings mean that the light rays reaching the helmet wearer""s eyes pass through the walls of the visors at angles far from the normal and, on passing through these walls, they undergo refraction phenomena causing offsets and image distortions.
Thus, the helmet wearer notices both that the image of the scenery which he can see is offset upward and that this image is distorted when he puts a visor in place.
The offset introduced by a visor varies with the direction in which the helmet wearer looks, since the value of the angle of offcentering depends on this direction. It is about 0.4xc2x0 when he looks straight ahead and rapidly gets worse when he looks sideways or downward. This offset has the drawback of playing on the superposition perceived by the helmet wearer between a synthetic projected image and the scenery that he can see.
Hitherto, the offset and the image distortions due to visors were ignored, the offset since it had no significant effect on the accuracy of the designation systems using a helmet visor and the image distortions since the helmet wearer can be satisfied most of the time with a reduced visual space by orienting his field of view by displacing his head rather than by displacing his eyes.
With the improvement in accuracy of the designation helmet visor systems, the effect of image offsets due to the presence of the visors on the sighting errors can no longer be ignored.
The offset introduced into the viewing axis by the protective visor or clear visor is easy to take into account. This is because, since this visor is generally kept in the lowered position during the whole mission, it is possible to take into account once and for all the offset that it introduces by an offset in the same direction of the synthetic projected image.
The offset introduced into the viewing axis by the additional optical protective visors is more problematic since the helmet wearer tends to place or retract these visors depending on the lighting conditions encountered or on the laser threats which may be very variable during one and the same mission. If one attempts to compensate for it by offsetting the synthetic projected image in the same direction, this can only be done when these visors are lowered, which means that the positions of these visors must be detectable at all times. This position detection can be obtained by equipping the helmet with visor position sensors, but that is against the general concern of lightening the helmet.
In addition, compensating for the offset of the scenery due to the presence of visors by a similar offset of the synthetic projected image has to be done according to the viewing direction of the helmet wearer since the value of the offcentering angle and therefore of the offset depends thereon. It would be possible to consider detecting the ocular position of the helmet wearer and offsetting the synthetic image by the value observed in the direction sighted by the helmet wearer, but this dynamic correction of the synthetic image is difficult to receive by the helmet wearer who sees a synthetic image which seems to undulate with the direction in which he looks.
Compensation only of the offset means that the image distortion due to the visors is not corrected, which is moreover particularly problematic for night or infrared viewing systems since it leads to poor superposition of the synthetic image with the scenery perceived through the visor. In order to improve this superposition, it would be possible to consider making the synthetic image subject to a similar distortion by image processing, but that would require the introduction of much too heavy optical or digital correction means on the helmet.
The aim of the present invention is to combat the aforementioned drawbacks by acting on the optical properties of the protective visors.
The subject of the invention is a protective visor for a helmet with a transparent wall having two faces, one internal, the other external, of generally spherical shape remarkable in that it comprises optical means for reducing angular offsets perceived through it by the helmet wearer.
Advantageously, said optical correction means consist of a relative axial displacement between the two spherical faces of the visor wall giving a variable thickness to this wall and leading to a prismatic effect correcting the angular offset perceived by the helmet wearer when he looks through the visor in a predetermined direction.
Advantageously, said optical correction means consist of a relative axial displacement of the external face of the visor with respect to the inner face in a vertical plane.
Advantageously, said optical correction means consist of an axial displacement upward through an angle of about half a degree of the outer face of the visor with respect to the inner face of the visor.
Advantageously, said optical correction means consist of reducing the ratio of the radius of curvature of the outer face of the visor to the radius of curvature of the inner face of the visor compared to that of a concentric bispherical visor of constant thickness.
Advantageously, said optical correction means consist of varying the ratio of the radius of curvature of the outer face to that of the inner face of the visor by about {fraction (1/100)}th compared to a concentric bispherical visor of constant thickness.
Advantageously, said optical correction means consist of adopting a spherical face and an aspherical face for the visor, the aspherical face approaching a theoretical shape minimizing the distortions, obtained by calculation on the basis of a spherical shape approximated by polygonal facets, each facet being reoriented so as to correct the angular offset that it imposes on the light rays reaching the helmet wearer""s eyes, the reoriented facets being repositioned in depth, each one with respect to the others, starting with the center of the visor and finishing at the edges, in order to obtain a surface having minimum roughness and the faceted surface obtained being smoothed by the calculation.
Advantageously, said aspherical face approaching a theoretical shape minimizing the distortions, obtained by the calculation, is of the toric type.
Advantageously, said optical correction means consist of adopting, for the visor, two aspherical faces obtained in two steps on the basis of two initially spherical faces, a first aspherical face being determined during a first step by approximating a first theoretical shape minimizing the distortions inherent to the initial spherical shapes, this first theoretical shape being obtained by calculation on the basis of a spherical shape approximated by polygonal facets, each facet being reoriented so as to correct the angular offset that it imposes on the light rays reaching the helmet wearer""s eyes, the reoriented facets being repositioned in depth, each one with respect to the others, starting with the center of the visor and finishing at the edges, in order to obtain a surface with minimum roughness and the faceted surface obtained being smoothed by the calculation, and a second aspherical face being obtained during a second step by approximating a second theoretical shape minimizing the distortions inherent to the spherical-aspherical shape obtained during the first step, this second theoretical shape being obtained by calculation on the basis of a spherical shape approximated by polygonal facets, each facet being reoriented so as to correct the angular offset that it imposes on the light rays reaching the helmet wearer""s eyes, the reoriented facets being repositioned in depth, each one with respect to the others, starting with the center of the visor and finishing at the edges, in order to obtain a surface having minimum roughness and the faceted surface obtained being smoothed by the calculation.
Advantageously, said aspherical faces approaching theoretical shapes minimizing the distortions, obtained by calculation, are of the toric type.
Advantageously, said optical correction means consist of a relative axial displacement between the two spherical faces of the visor leading to a prismatic effect correcting the angular offset perceived by the helmet wearer when he looks through the visor in an arbitrary direction and of adopting a ratio of the radius of curvature of the outer face of the visor to the radius of curvature of the inner face of the visor which is less than that of a concentric bispherical visor of constant thickness.
Advantageously, said optical correction means consist of a relative axial displacement between the two faces of the visor leading to a prismatic effect correcting the angular offset perceived by the helmet wearer when he looks through the visor in an arbitrary direction and of adopting, for the visor, a spherical face and an aspherical face configured, relative to the spherical face, so as to reduce the residual angular offsets in a region of the visor centered on said arbitrary direction.
Advantageously, said optical correction means consist of a relative axial displacement between the two faces of the visor leading to a prismatic effect correcting the angular offset perceived by the helmet wearer when he looks through the visor in an arbitrary direction and of adopting, for the visor, a spherical face and an aspherical face of the toric type configured, relative to the spherical face, so as to reduce the residual angular offsets in a region of the visor centered on said arbitrary direction.
Advantageously, said optical correction means consist of a relative axial displacement between the two faces of the visor leading to a prismatic effect correcting the angular offset perceived by the helmet wearer when he looks through the visor in an arbitrary direction and of adopting, for the visor, two aspherical faces configured one with respect to the other so as to reduce the residual angular offsets in a region of the visor centered on said arbitrary direction.
Advantageously, said optical correction means consist of a relative axial displacement between the two faces of the visor leading to a prismatic effect correcting the angular offset perceived by the helmet wearer when he looks through the visor in an arbitrary direction and of adopting, for the visor, two aspherical faces of the toric type configured, one with respect to the other, so as to reduce the residual angular offsets in a region of the visor centered on said arbitrary direction.
Advantageously, said optical correction means consist of a holographic diffraction mask affixed to one of the faces of the visor and determined so as to equalize the direction, at every point of impact on the visor, of the light rays collected by the helmet wearer""s eyes, before and after they have passed through the visor.
Advantageously, the holographic diffraction mask is affixed to the external face of the visor.
Advantageously, the holographic diffraction mask is assigned to the color green.